Heat exchanger and water heater

ABSTRACT

The heat exchanger including a plurality of fluid flow paths ( 401 ), ( 402 ), ( 403 ) arranged in a plurality of stages in a height direction, wherein a fluid inlet port ( 13   a ) and a fluid outlet port ( 13   b ) of the fluid flow path ( 403 ) located at a lowermost stage among the plurality of fluid flow paths ( 401 ), ( 402 ), ( 403 ) have larger flow path cross-sectional areas than a fluid inlet port ( 11   a ) and a fluid outlet port ( 11   b ) of the fluid flow path ( 401 ) located at an uppermost stage.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims a priority based on a Japanese Patent Application No. 2017-26634 filed on Feb. 16, 2017, the content of which is hereby incorporated by reference in its entirely.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a heat exchanger configured to heat a fluid to be heated flowing through a plurality of fluid flow paths arranged in a plurality of stages in a height direction by heat exchange with combustion exhaust gas and a water heater including the heat exchanger.

Description of the Related Art

Conventionally, there has been known a water heater having a can body in which a sub heat exchanger as a latent heat exchanger, a main heat exchanger as a sensitive heat exchanger, and a gas burner are disposed in this order from above (For example, Japanese Unexamined Patent Publication No. 2002-327960 A). In this type of water heater, as a part of a fluid flow path through which a fluid to be heated flows, a coiled water pipe is provided along side walls of the can body between a main heat transfer pipe of the main heat exchanger and the gas burner.

The fluid to be heated such as water supplied from a water supply pipe flows from a sub heat transfer pipe of the sub heat exchanger to the main heat transfer pipe of the main heat exchanger through the coiled water pipe. During the fluid to be heated flows through the fluid flow path, the fluid to be heated is heated by heat exchange with combustion exhaust gas ejected from the gas burner, and a heated fluid is supplied to a supply terminal through a hot-water supplying pipe connected to the main heat transfer pipe.

In the water heater described above, a single coiled water pipe is wound along the side walls of the can body, whereby abnormal overheating of the side walls is prevented. Further, in order to smoothly discharge the fluid to be heated from the fluid flow path in drainage work for preventing freezing, when the water heater can use a can body having a sufficient height, the coiled water pipe inclined at a predetermined degree is wound around the side walls of the can body (for example, at about 5 degrees with respect to the horizontal).

On the other hand, there has also been proposed a so-called downward combustion-type water heater in which a heat exchanger is provided below a gas burner having a downward combustion surface on the contrary to such disposition of the heat exchanger and the gas burner as described above (For example, Japanese Unexamined Patent Publication No. 2016-169934 A).

In order to reduce flow resistance by the coiled water pipe, the above-described downward combustion-type water heater includes a main heat exchanger having a single distribution header connecting with a plurality of fluid flow paths and a single collection header connecting with the plurality of fluid flow paths.

The plurality of fluid flow paths have straight heat transfer pipes arranged in a plurality of stages in a height direction along two opposite side walls of a case body. Moreover, fluid inlet ports and fluid outlet ports of the plurality of fluid flow paths are connected to the single distribution header and the single collection header, respectively.

In order to reduce a height of the main heat exchanger having such a configuration, each of the fluid flow paths is needed to be arranged in a substantially horizontal posture. However, in a case where the respective fluid flow paths are provided substantially horizontally, the fluid inlet port and the fluid outlet port of each of the fluid flow paths are located at substantially the same height. Therefore, drainage performance for water from the fluid flow path deteriorates.

In particular, the water is hardly discharged from the fluid flow path located at a lowermost stage among the fluid flow paths arranged in the plurality of stages, whereby the water tends to remain in the fluid flow path located at the lowermost stage. As a result, when an outside air temperature drops to 0 degrees Celsius or less in winter in a state in which the water remains in the fluid flow paths, residual water freezes and a volume of the residual water expands, which may result in damaging the fluid flow paths.

SUMMARY OF THE INVENTION

The present invention has been made in view of the foregoing problems, and an object of the present invention is to provide a heat exchanger capable of smoothly and reliably discharging a fluid to be heated from a plurality of fluid flow paths even when the plurality of fluid flow paths are arranged in a plurality of stages in a height direction along a side wall of a case body, and fluid inlet ports and fluid outlet ports of the plurality of fluid flow paths are connected to a single distribution header and a single collection header, respectively, and to provide a water heater using the heat exchanger.

According to one aspect of the present invention, there is provided a case body including a flow passage of combustion exhaust gas therein;

a plurality of fluid flow paths arranged in a plurality of stages in a height direction along a side wall of the case body;

a distribution header communicating with fluid inlet ports of the plurality of fluid flow paths and distributing a fluid to be heated to the plurality of fluid flow paths; and

a collection header communicating with fluid outlet ports of the plurality of fluid flow paths and collecting the fluid to be heated from the plurality of fluid flow paths, wherein

the fluid inlet port and the fluid outlet port of the fluid flow path located at a lowermost stage among the plurality of fluid flow paths have larger flow path cross-sectional areas than the fluid inlet port and the fluid outlet port of the fluid flow path located at an uppermost stage among the plurality of fluid flow paths, respectively.

According to another aspect of the present invention, there is provided a water heater having the heat exchanger described above.

In accordance with the present invention, the fluid to be heated can be discharged smoothly and reliably even in the heat exchanger in which the plurality of fluid flow paths are arranged in the plurality of stages in the height direction along the side wall of the case body, and the fluid inlet ports and the fluid outlet ports of the plurality of fluid flow paths are connected to the single distribution header and the single collection header, respectively. Therefore, a height of the case body can be reduced as compared with a heat exchanger in which a heat transfer pipe is inclinedly wound around the side walls of the case body. Hence, the heat exchanger suitably used for a downward combustion-type water heater can be provided.

Moreover, since the water heater having the above-described heat exchanger is excellent in drainage performance for water, even in winter when an outside air temperature drops, freezing and expansion of the fluid remaining in the fluid flow path hardly occur. Therefore, rupture or damage of a heat transfer pipe constituting the fluid flow path hardly occurs. Hence, the water heater having excellent durability can be provided.

Other objects, features and advantages of the present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram showing one example of a water heater including a heat exchanger according to an embodiment of the present invention;

FIG. 2 is a schematic partial exploded perspective view showing one example of the heat exchanger according to the embodiment of the present invention;

FIG. 3 is a schematic partial enlarged cross-sectional view showing one example of the heat exchanger according to the embodiment of the present invention; and

FIG. 4 is schematic partial enlarged front views showing heat exchangers according to other embodiments of the present invention, wherein FIG. 4A shows a heat exchanger in which a plurality of fluid flow paths are arranged in such a manner that flow path cross-sectional areas of a fluid inlet port and a fluid outlet port become larger in order from above, respectively, FIG. 4B shows a heat exchanger in which a plurality of fluid flow paths are arranged in such a manner that fluid inlet ports and fluid outlet ports of the fluid flow paths located at middle and lowermost stages have the same flow path cross-sectional areas, respectively, and that the fluid inlet ports and the fluid outlet ports of the fluid flow paths located at the middle and lowermost stages have larger flow path cross-sectional areas than a fluid inlet port and a fluid outlet port of the fluid flow path located at an uppermost stage, respectively, and FIG. 4C shows a heat exchanger in which a plurality of fluid flow paths are arranged in such a manner that flow path cross-sectional areas of the plurality of fluid flow paths become larger in order of an uppermost, a lowermost, and middle stages.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, referring to drawings, a heat exchanger according to an embodiment of the present invention will be described in detail.

The heat exchanger (3) according to the present embodiment is incorporated in a water heater (4). As shown in FIG. 1, in the water heater (4), a gas burner (50) having a downward combustion surface is disposed in an upper region of the water heater (4). Moreover, a lower portion of the gas burner (50) is continuously connected to a substantially rectangular box-shaped case body (30) of a heat exchanger (3). Furthermore, a lower portion of the case body (30) is continuously connected to an exhaust passage (31) guiding combustion exhaust gas supplied from the gas burner (50) to an outside of the water heater (4).

An upper portion of the gas burner (50) is connected to a fan unit (5) supplying air outside the water heater (4) as combustion air for the gas burner (50). The combustion exhaust gas ejected from the gas burner (50) is fed into the heat exchanger (3) by the fan unit 5. Then, the combustion exhaust gas is discharged to the outside of the water heater (4) through the exhaust passage (31).

Water, as a fluid to be heated, supplied from a water supply pipe (41) flows into second heat transfer pipes (2) located in a lower half region of the case body (30) of the heat exchanger (3), and subsequently, flows into first heat transfer pipes (1) as coiled water pipes located in an upper half region of the case body (30). When the water sequentially flows through the second heat transfer pipes (2) and the first heat transfer pipes (1), the water is heated by heat exchange with the combustion exhaust gas ejected from the gas burner (50), and hot water as a heated fluid is supplied to a hot water supply terminal through a hot-water supply pipe (42).

Acid drain generated on surfaces of heat transfer fins (33) is collected by a drain receiver (40) to discharge to the outside of the water heater (4) through a drain neutralizer (not shown) from a drain pipe (43).

As shown in FIG. 2, between front and back facing side walls (301), (302) of the case body (30), a plurality of plate-shaped heat-transfer fins (33) made of stainless steel-based metal are provided vertically, and arranged side horizontally by side at predetermined intervals in substantially parallel with the front and back side walls (301), (302). Only some heat-transfer fins (33) are shown in FIG. 2 to avoid over-crowding the drawing.

Furthermore, a plurality of first heat transfer pipes (1) made of stainless steel-based metal and a plurality of second heat transfer pipes (2) made of stainless steel-based metal are individually extended so as to bridge between the front and back facing side walls (301), (302) of the case body (30). The second heat transfer pipes (2) are disposed so as to penetrate the heat-transfer fins (33).

In the following description of the present specification, an outer surface of the front side wall (301) corresponds to a front of the heat exchanger (3), a depth direction as viewed from a front side of the case body (30) corresponds to a front-and-back direction, and a width direction and a height direction as similarly viewed correspond to a left-and-right direction and an up-and-down direction, respectively.

The plurality of second heat transfer pipes (2) (here, eight) are arranged substantially in the lower half region of the case body (30). Each of the second heat-transfer pipes 33 is made of a straight pipe having a substantially elliptical cross-sectional shape. The number of second heat transfer pipes (2) can be selected as appropriate depending on a configuration of the heat exchanger (3).

In FIG. 2, two upstream open ends (2 a) disposed adjacent in the left-and-right direction open at a left end of the front side wall (301), two downstream open ends (2 b) disposed adjacent in the left-and-right direction open at a right end of the front side wall (301), and four open ends (not shown) open at a middle portion of the front side wall (301). The two upstream open ends (2 a), the two downstream open ends (2 b), and the four open ends are configured to communicate with one another via an inflow header (21) connected to the water supply pipe (41), via a distribution header (22) connected to the first heat transfer pipes (1), and via an intermediate header (23), respectively. Although not shown, on the back side wall (302), two left and right connection headers are provided in such a manner that eight open ends of the second heat transfer pipes (2) on a back side wall (302) side communicate four by four with one another. Thereby, a lower fluid flow path (410) is formed in the case body (30).

Along substantially upper halves of left and right side walls (303), (304) in the case body (30), the plurality of first heat transfer pipes (1) (here, six) are arranged substantially horizontally. Each of the first heat-transfer pipes (1) is made of a straight pipe having a substantially circular cross-sectional shape. The number of first heat transfer pipes (1) can be selected as appropriate depending on the configuration of the heat exchanger (3).

In FIG. 2, three heat transfer pipes (11), (12), (13) located at uppermost, middle, and lowermost stages are arranged along the right side wall (304), and upstream open ends (11 a), (12 a), (13 a) thereof open at the right end of the front side wall (301) and communicate with the downstream open ends (2 b) of the second heat transfer pipes (2) via the distribution header (22). Moreover, another three heat transfer pipes (11), (12), (13) located at uppermost, middle, and lowermost stages are arranged along the left side wall (303), and downstream open ends (11 b), (12 b), (13 b) thereof open at the left end of the front side wall (301) and communicate with the collection header (22) connected to the hot-water supply pipe (42).

Although not shown, on the back side wall (302), there is provided one back-side connection header configured to connect the open ends of the left and right heat transfer pipes (11) located at the uppermost stage. Further, on the back side wall (302), there is provided another back-side connection header configured to connect the open ends of the left and right heat transfer pipes (12), (13) located at the middle and lowermost stages.

Thereby, upper fluid flow paths (401), (402), (403) are formed at three stages in the up-and-down direction in such a manner that the water supplied from the lower fluid flow path (410) flows in parallel in a substantially U-shape along substantially the upper halves of the back, left, and right side walls (302), (303), (304) in the case body (30). Accordingly, the three upstream open ends (11 a), (12 a), (13 a) of the first heat transfer pipes (1) disposed along the right side wall (304) and connected to the distribution header (22) constitute fluid inlet ports of the respective upper fluid flow paths (401), (402), (403). Moreover, the three downstream open ends (11 b), (12 b), (13 b) of the first heat transfer pipes (1) disposed along the left side wall (303) and connected to the collection header (24) constitute fluid outlet ports of the respective upper fluid flow paths (401), (402), (403).

In the present embodiment, the left and right first heat transfer pipes (11), (12), (13) are arranged in such a manner that opening areas (for example, inner diameter: 14 mm) of the upstream open ends (13 a) and downstream open ends (13 b) of the heat transfer pipes (13) located at the lowermost stage are larger than opening areas (for example, inner diameter: 11 mm) of the upstream open ends (11 a), (12 a) and downstream open ends (11 b), (12 b) of the heat transfer pipes (upper-stage heat transfer pipes) (11), (12) located at the uppermost and middle stages (an upper stage as viewed from the lowermost stage), respectively.

Moreover, the heat transfer pipes (13) located at the lowermost stage are disposed so as to protrude more inwardly of the case body (30) in the left-and-right direction than the upper-stage heat transfer pipes (11), (12) located at the upper stage.

The respective headers (21) to (24) include header bodies (21 a) to (24 a) and header covers (21 b) to (24 b). Each of the header bodies (21 a) to (24 a) is formed by depressing a part of the front side wall (301) inward by subjecting drawing to a predetermined portion of the front side wall (301) of the case body (30), and each of the header covers (21 b) to (24 b) is connected in a watertight state to a peripheral edge of each of the header bodies (21 a) to (24 a). This allows an internal space in a predetermined volume to be formed between a depressed bottom surface of each of the header bodies (21 a) to (24 a) and a back surface of each of the header cover (21 b) to (24 b).

The upstream open ends (2 a) of the second heat transfer pipes (2), the downstream open ends (2 b) of the second heat transfer pipes (2) and the upstream open ends (11 a) to (13 a) of the first heat transfer pipe (1), and the downstream open ends (11 b) to (13 b) of the first heat transfer pipe (1) are open to internal spaces of the headers (21), (22), (24), respectively. Namely, the respective headers (21), (22), (24) communicate with the upstream open ends (2 a), the downstream open ends (2 b) and the upstream open ends (11 a) to (13 a), the downstream open ends (11 b) to (13 b).

In the heat exchanger (3) of the present embodiment, the plurality of first heat transfer pipes (1) are arranged in the case body (30). Since these first heat transfer pipes (1) are arranged in a plurality of stages (here, three stages) in the up-and-down direction (the height direction) along the left and right side walls (303), (304), heat of the combustion exhaust gas at a high temperature introduced into the case body (30) from the gas burner (50) located above is efficiently absorbed by the plurality of first heat transfer pipes (1). Therefore, abnormal overheating of the left and right side walls (303), (304) is suppressed.

Moreover, while the first heat transfer pipes (1) are arranged in three stages in the up-and-down direction, the heat transfer pipes (13)located at the lowermost stage are arranged so as to protrude more inwardly of the case body (30) than the upper-stage heat transfer pipes (11), (12). Therefore, it is possible not only to efficiently heat the heat transfer pipes (13) located at the lowermost stage by the combustion exhaust gas flowing in the case body (30) but also to flow the combustion exhaust gas inwardly of the case body (30). As a result, even in a case where no first heat transfer pipe (1) is disposed along the lower halves of the left and right side walls (303), (304) of the case body (30), the abnormal overheating of the left and right side walls (303), (304) can be prevented reliably. Further, the water flowing through the respective upper fluid flow paths (401), (402), (403), is efficiently heated by heat exchange with the combustion exhaust gas.

When combustion operation is performed by the water heater (4) of the present embodiment, the water supplied from the water supply pipe (41) into the heat exchanger (3) flows into the second heat transfer pipes (2) from a water supply port (20) provided on the inflow header (21) located at a left front lower end of the case body (30).

Subsequently, the water flowing through the second heat transfer pipes (2) is supplied into the internal space of the distribution header (22) from the downstream open ends (2 b) located at a right front lower end of the case body (30). Then, the water flows from the upstream open ends (11 a), (12 a), (13 a) to the first heat transfer pipes (1) arranged along the right side wall (304). Moreover, the water flowing through the respective upper fluid flow paths (401), (402), (403) flows out from the respective downstream open ends (11 b), (12 b), (13 b), which open at a left-end upper region of the front side wall (301) of the case body (30), to the internal space of the collection header (24) provided from the left-end upper region of the front side wall (301) of the case body (30) to a central region thereof. The water that has flown out to the collection header (24) is discharged to the hot-water supply pipe (42) via a hot-water supply port (25) provided in the collection header (24).

On the other hand, when drainage work is performed in the water heater (4) of the present embodiment, the water is discharged from the lower water supply port (20). Then, following discharging of the water from the water supply port (20), the water remaining in the first and second heat transfer pipes (1), (2) flows reversely through the first and second heat transfer pipes (1), (2) toward the upstream open ends (2 a) of the second heat transfer pipes (2).

Moreover, following discharging of the water, the water filling the internal spaces of the distribution header (22) and the collection header (24) decreases, and water levels in the internal spaces thereof drop. When the water level drops below the hot-water supply port (25) of the collection header (24), air enters the collection header (24) from the hot-water supply port (25), and a volume of the discharged water is replaced with air.

As shown in FIG. 3, among the plurality of first heat transfer pipes (1), the downstream open end (11 b) of the heat transfer pipe (11) located at the uppermost stage is first opened to an air layer in the collection header (24). Then, as shown by an arrow 1, as the water is discharged from the upstream open end (11 a) (see FIG. 2), the air flows into the heat transfer pipe (11) located at the uppermost stage. As described above, since the water in the heat transfer pipe (11) located at the uppermost stage is preferentially discharged, the water hardly remains in the heat transfer pipe (11) located at the uppermost stage.

Subsequently, as the water level in the distribution header (22) further drops, the water in the heat transfer pipe (12) located at the middle stage is also discharged from the upstream open end (12 a) (see FIG. 2). Then, as shown by an arrow 2, the air flows into the heat transfer pipe (12) located at the middle stage from the downstream open ends (12 b).

Finally, the water is discharged from the upstream open end (13 a) of the heat transfer pipe (13) located at the lowermost stage (see FIG. 2). Although the heat transfer pipe (13) is communicated with the internal space of the collection header (24), the downstream open end (13 b) is open to a lower region in the collection header (24). Therefore, all the water in the heat transfer pipe (13) located at the lowermost stage is hardly replaced with air, and the water tends to remain in the heat transfer pipe (13).

In particular, as compared with the heat transfer pipes (11), (12) located at the uppermost and middle stages, in the heat transfer pipe (13) located at the lowermost stage, a water head pressure is lowered by discharging of the water up to that time. Therefore, the water is hardly discharged by surface tension acting on the upstream open end (13 a).

However, since a flow path cross-sectional area of the upstream open end (13 a) of the heat transfer pipe (13) located at the lowermost stage is set larger than those of the upstream open ends (11 a), (12 a) of the heat transfer pipes (11), (12) located at the uppermost and middle stages, an amount of the water in the heat transfer pipes (13) located at the lowermost stage is larger than those of the heat transfer pipes (11), (12) located at the uppermost and middle stages. Therefore, during the drainage work, the water head pressure in the heat transfer pipes (13) can be increased to such an extent of overcoming the surface tension acting on the upstream open end (13 a). As a result, as indicated by an arrow 3, the air flows into the heat transfer pipes (13), and the water remaining in the heat transfer pipes (13) located at the lowermost stage can be discharged smoothly. Further, since the second heat-transfer pipes (2) are made of a straight pipe having the substantially elliptical cross-sectional shape excellent in drainage performance, the water in the second heat-transfer pipes (2) can be also discharged smoothly.

In accordance with the above-described embodiment, even in a case where the three heat transfer pipes (11) to (13) located at the uppermost, middle, and lowermost stages are arranged substantially horizontally along each of the left and right side walls (303), (304) of the case body (30), it is possible to promptly discharge the water from all of the heat transfer pipes (11) to (13). Therefore, even in winter when an outside air temperature drops to 0 degrees Celsius or less, it is possible to prevent damage of the first and second heat transfer pipes (1), (2) due to freezing and expansion of the water remaining in the first and second heat transfer pipes (1), (2) of the heat exchanger (3).

Moreover, in accordance with the above-described embodiment, the water can be discharged smoothly even if the heat transfer pipes (11) to (13) are arranged substantially horizontally in the heat exchanger (3). Therefore, the height of the case body (30) can be reduced as compared with such a heat exchanger in which an inclined coiled water pipe is wound around the side walls of the case body (30). Hence, the heat exchanger (3) can be suitably incorporated in the water heater (4) as shown in FIG. 1.

In the above-described embodiment, as shown in FIGS. 2 and 3, the straight pipes having the upstream open ends (11 a), (12 a) and downstream open ends (11 b), (12 b) with the same small flow path cross-sectional area are used as the heat transfer pipes (11), (12) located at the uppermost and middle stages, and the straight pipes having the upstream open ends (13 a) and downstream open ends (13 b) with the larger flow path cross-sectional areas than the upstream open ends (11 a), (12 a) and downstream open ends (11 b), (12 b) of the heat transfer pipes (11), (12) are used as the heat transfer pipes (13) located at the lowermost stage.

However, the present invention is not limited to such an arrangement form of the heat transfer pipes (11), (12), (13). As long as the upstream open end (13 a) and downstream open end (13 b) of the heat transfer pipe (13) located at the lowermost stage have larger flow path cross-sectional areas than the upstream open end (11 a) and downstream open end (11 b) of the heat transfer pipes (11) located at the uppermost stage, respectively, the present invention can also be applied to other arrangement form of the heat transfer pipes (11), (12), (13) as described below.

For example, as shown in FIG. 4A, the plurality of first heat transfer pipes (11), (12), (13) may be arranged in such a manner that the flow path cross-sectional areas of the upstream open ends (11 a), (12 a), (13 a) and the downstream open ends (11 b), (12 b), (13 b) become larger in order from the top, respectively.

Moreover, for example, as shown in FIG. 4B, the plurality of heat transfer pipes (11), (12), (13) may be arranged in such a manner that the upstream open ends (12 a), (13 a) and downstream open ends (12 b), (13 b) of the heat transfer pipes (12), (13) located at the middle and lowermost stages (a lower stage as viewed from the uppermost stage) have the same flow path cross-sectional area, and that the above-described flow path cross-sectional area of the upstream open ends (12 a), (13 a) and downstream open ends (12 b), (13 b) of the lower-stage heat transfer pipes (12), (13) becomes larger than that of the upstream open end (11 a) and downstream open end (11 b) of the heat transfer pipe (11) located at the uppermost stage.

Furthermore, for example, as shown in FIG. 4C, the plurality of heat transfer pipes (11), (12), (13) may be arranged in such a manner that the upstream open end (12 a) and downstream open ends (12 b) of the heat transfer pipes (12) located at the middle stage have the largest flow path cross-sectional areas as long as the flow path cross-sectional areas of the upstream open end (13 a) and downstream open end (13 b) of the heat transfer pipe (13) located at the lowermost stage are larger than those of the heat transfer pipe (11) located at the uppermost stage, respectively.

In the above-described embodiment, straight pipes having the upstream open end (13 a) and the downstream open end (13 b) with the same flow path cross-sectional area are used as the heat transfer pipes (13) located at the lowermost stage. However, only openings of the upstream open end (13 a) and the downstream open end (13 b) just need to be largely expanded in diameter, and a pipe diameter of an intermediate portion of the fluid flow path (403) other than both of the open ends (13 a), (13 b) may be the same as that of the heat transfer pipes (11) located at the uppermost stage or the heat transfer pipes (12) located at the middle stage. Even if the heat transfer pipes (13) located at the lowermost stage have such a shape, the surface tension acting on the upstream open end (13 a) and the downstream open end (13 b) decreases by using the heat transfer pipes (13) having the upstream open end (13 a) and the downstream open end (13 b), each having a large flow path cross-sectional area. In this way, the water in the heat transfer pipes (13) located at the lowermost stage can be discharged smoothly.

Moreover, the first heat transfer pipes (1) constituting the upper fluid flow paths (401), (402), (403) are not limited to straight pipes. For example, substantially U-shaped continuous pipes in which the upstream open ends (11 a), (12 a), (13 a) and the downstream open ends (11 b), (12 b), (13 b) only open at the front side wall (301) may be used. Further, the upper fluid flow paths (401), (402), (403) may be arranged along only either one of the left and right side walls (303), (304). Furthermore, the number of stages is not limited to three stages. The plurality of fluid flow paths may be arranged in two stages or arranged in four or more stages.

Meanwhile, the water heater may have a sub heat exchanger below the heat exchanger (3).

As described above in detail, the present invention is summarized as follows.

According to one aspect of the present invention, there is provided a heat exchanger comprising:

a case body including a flow passage of combustion exhaust gas therein;

a plurality of fluid flow paths arranged in a plurality of stages in a height direction along a side wall of the case body;

a distribution header communicating with fluid inlet ports of the plurality of fluid flow paths and distributing a fluid to be heated to the plurality of fluid flow paths; and

a collection header communicating with fluid outlet ports of the plurality of fluid flow paths and collecting the fluid to be heated from the plurality of fluid flow paths, wherein

the fluid inlet port and the fluid outlet port of the fluid flow path located at a lowermost stage among the plurality of fluid flow paths have larger flow path cross-sectional areas than the fluid inlet port and the fluid outlet port of the fluid flow path located at an uppermost stage among the plurality of fluid flow paths, respectively.

According to the heat exchanger described above, the fluid to be heated flows into the plurality of fluid flow paths from the fluid inlet ports of the plurality of fluid flow paths arranged in the plurality of stages in the height direction along the side wall of the case body, flows through the plurality of fluid flow paths, and then flows out from the fluid outlet ports into the collection header. The plurality of fluid flow paths are arranged in the plurality of stages along the side wall of the case body through which the combustion exhaust gas flows. Therefore, heat of the combustion exhaust gas is efficiently absorbed by the plurality of fluid flow paths and abnormal overheating of the side wall is suppressed.

Moreover, for example, when drainage work is performed in the heat exchanger in which a water supply port is disposed below a hot-water supply port, the fluid flowing reversely toward an upstream side through the fluid flow paths is discharged from the water supply port at a lower position.

At the same time, air flows into the fluid flow paths from the hot-water supply port at an upper position. Following discharging of the fluid to be heated, the fluid to be heated filling the internal spaces of the distribution header and the collection header decreases, so that the air flows into the fluid flow paths from the fluid outlet ports being open to the collection header and a volume of the discharged fluid is replaced with air. At this time, since a plurality of fluid outlet ports are arranged in the plurality of stages in the height direction in the connection header, the air preferentially flows into the fluid outlet port of the fluid flow path located at the uppermost stage, which is open to the collection header. Therefore, the fluid to be heated in the fluid flow path located at the uppermost stage is relatively easily discharged.

On the other hand, a water head pressure in the fluid flow path located at the lowermost stage is lowered by discharging the fluid up to that time. Therefore, the fluid to be heated is held by surface tension acting on the fluid inlet port of the fluid flow path located at the lowermost stage, whereby the fluid to be heated is extremely hardly discharged.

However, according to the heat exchanger described above, since the flow path cross-sectional areas of the fluid inlet port and the fluid outlet port of the fluid flow path located at the lowermost stage are set larger than those of the fluid inlet port and the fluid outlet port of the fluid flow path located at the uppermost stage, respectively, an amount of the fluid to be heated in the fluid flow path located at the lowermost stage is larger than that of fluid flow path located at the uppermost stage. As a result, the water head pressure in the fluid flow path located at the lowermost stage can be increased to such an extent of overcoming the surface tension acting on the fluid inlet port of the fluid flow path located at the lowermost stage, whereby the fluid to be heated remaining in the fluid flow path located at the lowermost stage can be discharged smoothly.

Preferably, in the heat exchanger described above,

the fluid inlet port and the fluid outlet port of a lower-stage fluid flow path located at a lower stage than the fluid flow path located at the uppermost stage among the plurality of fluid flow paths have larger flow path cross-sectional areas than the fluid inlet port and the fluid outlet port of the fluid flow path located at the uppermost stage, respectively.

According to the heat exchanger described above, since the fluid inlet port and the fluid outlet port of the lower-stage fluid flow path located at the lower stage than the fluid flow path located at the uppermost stage have larger flow path cross-sectional areas than the fluid inlet port and the fluid outlet port of the fluid flow path located at the uppermost stage, respectively, the amount of the fluid to be heated in the lower-stage fluid flow path becomes larger than that in the uppermost fluid flow path. Therefore, the water remaining in the lower-stage fluid flow path including the fluid flow path located at the lowermost stage can be discharged smoothly.

Preferably, in the heat exchanger described above,

the fluid inlet port and the fluid outlet port of the fluid flow path located at the lowermost stage among the plurality of fluid flow paths have larger flow path cross-sectional areas than the fluid inlet port and the fluid outlet port of an upper-stage fluid flow path located at a upper stage than the fluid flow path located at the lowermost stage, respectively.

As described above, the fluid to be heated in the fluid flow path located at the lowermost stage is extremely hardly discharged.

However, according to the heat exchanger described above, since the fluid inlet port and the fluid outlet port of the fluid flow path located at the lowermost stage have larger flow path cross-sectional areas than the fluid inlet port and the fluid outlet port of the upper-stage fluid flow path located at the upper stage than the fluid flow path located at the lowermost stage, respectively, the amount of the fluid to be heated in the fluid flow path located at the lowest stage becomes larger than that in the upper-stage fluid flow path. Therefore, the water remaining in the fluid flow path located at the lowermost stage can be discharged smoothly.

Preferably, in the heat exchanger described above,

the fluid flow path located at the lowermost stage among the plurality of fluid flow paths is disposed so as to protrude more inwardly of the case body than an upper-stage fluid flow path located at a upper stage than the fluid flow path located at the lowermost stage as viewed from an upstream side of the flow passage of the combustion exhaust gas.

According to the heat exchanger described above, it is possible not only to efficiently heat the fluid flow path located at the lowermost stage by the combustion exhaust gas flowing in the case body but also to flow the combustion exhaust gas inwardly of the case body. Therefore, the abnormal overheating of the side wall of the case body can be prevented reliably. Moreover, the fluid to be heated flowing through the fluid flow paths is efficiently heated by heat exchange with the combustion exhaust gas.

According to another aspect of the present invention, there is provided a water heater having the heat exchanger described above.

By use of the heat exchanger described above, it makes possible to enhance the drainage performance for water. Therefore, damage of the heat exchanger caused by freezing and expansion of the fluid to be heated remaining in the fluid flow path can be prevented reliably. Hence, the water heater having excellent durability can be provided.

Although the present invention has been described in detail, the foregoing descriptions are merely exemplary at all aspects, and do not limit the present invention thereto. It should be understood that an enormous number of unillustrated modifications may be assumed without departing from the scope of the present invention. 

What is claimed is:
 1. A heat exchanger comprising: a case body including a flow passage of combustion exhaust gas therein; a plurality of fluid flow paths arranged in a plurality of stages in a height direction along a side wall of the case body; a distribution header communicating with fluid inlet ports of the plurality of fluid flow paths and distributing a fluid to be heated to the plurality of fluid flow paths; and a collection header communicating with fluid outlet ports of the plurality of fluid flow paths and collecting the fluid to be heated from the plurality of fluid flow paths, wherein the fluid inlet port and the fluid outlet port of the fluid flow path located at a lowermost stage among the plurality of fluid flow paths have larger flow path cross-sectional areas than the fluid inlet port and the fluid outlet port of the fluid flow path located at an uppermost stage among the plurality of fluid flow paths, respectively.
 2. The heat exchanger according to claim 1, wherein the fluid inlet port and the fluid outlet port of a lower-stage fluid flow path located at a lower stage than the fluid flow path located at the uppermost stage among the plurality of fluid flow paths have larger flow path cross-sectional areas than the fluid inlet port and the fluid outlet port of the fluid flow path located at the uppermost stage, respectively.
 3. The heat exchanger according to claim 1, wherein the fluid inlet port and the fluid outlet port of the fluid flow path located at the lowermost stage among the plurality of fluid flow paths have larger flow path cross-sectional areas than the fluid inlet port and the fluid outlet port of an upper-stage fluid flow path located at a upper stage than the fluid flow path located at the lowermost stage, respectively.
 4. The heat exchanger according to claim 1, wherein the fluid flow path located at the lowermost stage among the plurality of fluid flow paths is disposed so as to protrude more inwardly of the case body than an upper-stage fluid flow path located at an upper stage than the fluid flow path located at the lowermost stage as viewed from an upstream side of the flow passage of the combustion exhaust gas.
 5. A water heater comprising the heat exchanger according to claim
 1. 